This invention relates generally to a method and apparatus for charging rechargeable batteries.
The several advantages of cordless power for portable power tools and certain kitchen and domestic appliances have led to the development of a wide range of sizes of power- or battery-packs, that is, a contained group of power cells. These power cells may include nickel cadmium (NiCd), nickel metal hydride (Nix), lithium, or lead-acid cells, etc.
Referring to FIGS. 1-2, a typcial battery pack 10 is connected to a charger 20. Battery pack 10 comprises a plurality of battery cells 11 connected in series, which dictate the voltage and storage capacity for battery pack 10. Battery pack 10 includes three battery contacts: first battery contact 12, second battery contact 14, and third battery contact 13. Battery contact 12 is the B+ (positive) terminal for battery pack 10. Battery contact 14 is the Bxe2x88x92 or negative/common terminal. Battery contact 13 is the S or sensing terminal. Battery contacts 12 and 14 receive the charging current sent from the charger 20 (preferably from current source 22, as discussed below) for charging the battery pack 10.
As shown in FIG. 2, the battery cells 11 are coupled between the battery contacts 12 and 14. In addition, a temperature sensing device 15, such as a negative temperature co-efficient (NTC) resistor, or thermistor, RT is typically coupled between battery contacts 13 and 14. The temperature sensing device is preferably in closer proximity to the cells 11 for monitoring of the battery temperature. Other components, such as capacitors, etc., or circuits can be used to provide a signal representative of the battery temperature.
The charger 20 preferably comprises a controller 21, which in turn includes positive terminal (B+) 16 and negative (Bxe2x88x92) terminal 17, which are coupled to battery pack 10 via battery contacts 12 and 14, respectively. The positive terminal may also act as an input, preferably an analog/digital input, in order for the controller 21 to detect the battery voltage. In addition, the controller 21 may include another input T, preferably an analog/digital input, which is coupled to the temperature sensing device 15 via the third battery contact 13 (S). This allows the controller 21 to monitor the battery temperature. Controller 21 includes a microprocessor 23 for controlling the charging and monitoring operations. Controller 21 may control a current source 22 that provides current to battery pack 10. This current may be a fast charging current and/or an equalization current. Current source 22 may be integrated within controller 21.
Referring to FIG. 3, the battery temperature and voltage varies during the charging process. for example, battery temperature decreases as the battery is charged. The battery temperature then quickly increases as the battery becomes fully charged. However, if the charging process is not stopped when the battery is fully charged, the battery could be overcharged and thus damaged by the rising temperature. Accordingly, battery temperature or battery voltage are usually monitored as indicators of the full charge condition.
Among the voltage monitoring methods, the Saar double inflection termination method described in U.S. Pat. Nos. 4,388,582 and 4,392,101, is preferred to detect a battery reaching full charge. Other voltage monitoring methods more typically employed are (1) the minus-delta-voltage method, (2) the peak detect method, and (3) the voltage slope detect method. In the minus-delta-voltage method, a sample of the battery peak voltage is stored and compared to the most recent voltage. Termination occurs when the most recent voltage falls below a set point, usually within between 0.5% and 1.0% of the stored peak, or about 10 to 20 millivolts per cell for a NiCd battery.
The peak detect method is more modem version of the minus-delta-voltage method. Basically, the same method is used, except the set point can be set closer to the peak by using more accurate instrumentation.
The slope detect method is another voltage monitoring method. According to this method, the voltage peak B is detected by calculating the slope of the voltage curve V, or voltage change rate (dV/dt). Termination occurs when the voltage change rate is 0 or negative.
Temperature monitoring methods typically employed are (1) absolute temperature termination and (2) temperature change rate (slope) termination. Absolute temperature termination relies on the temperature rise that occurs when the battery is fully charged. Under this method, the charging process will be stopped when the battery temperature reaches and/or exceeds a certain temperature.
The temperature change rate (slope) termination method requires monitoring the slope of the battery temperature over time, or temperature change rate (dT/dt), during the charging process. Termination occurs when the temperature change rate reaches and/or exceeds a predetermined rate. In other words, termination occurs when a trip point is reached and/or exceeded.
Referring to FIGS. 1-2, the charger 20 may accept different battery packs 10, 10xe2x80x2, 10xe2x80x3, where like numerals refer to like parts. Battery packs 10, 10xe2x80x2, 10xe2x80x3 are similar, but differ in several respects. First, both battery packs 10, 10xe2x80x2 receive air blown from charger fan 24 to cool cells 11. In battery pack 10, the temperature sensing device 15 is covered and/or disposed outside of the airflow, so that the airflow does not affect the temperature sensing. On the other hand, in battery pack 10xe2x80x2, the temperature sensing device 15 is not covered and/or disposed in of the airflow, so that the airflow affects the temperature sensing. Battery pack 10xe2x80x3 does not receive any air blown from charger fan 24. Accordingly, the temperature sensing device 15 cannot be affected by blown air.
FIG. 4 shows the temperature/voltage curves T and V, respectively, when battery pack 10xe2x80x2 is charged, as opposed to the temperature/voltage curves for battery packs 10, 10xe2x80x3, shown in FIG. 3. If the temperature curves T of FIGS. 3-4 are compared, it is obvious that the temperature curve of battery pack 10xe2x80x3 is not as smooth, with many peaks and valleys. This is because the airflow affects the sensed temperature. However, having such peaks and valleys can cause termination of the charging process under the temperature change rate termination scheme prior to fully charging battery pack 10xe2x80x2. In other words, battery pack 10xe2x80x2 may be undercharged because of the airflow.
It is an object of the invention to provide a charging and monitoring method that will not result in undercharged batteries.
In accordance with the present invention, a method for charging a rechargeable battery pack is proposed. The charging method includes providing a current to the battery pack, sensing first and second battery temperatures, determining a first temperature change rate between the first and second battery temperatures, sensing a third battery temperature, determining a second temperature change between the second and third battery temperatures, and disabling termination of the charging method based on a temperature-based scheme if the first temperature change rate is equal to or exceeds a first predetermined threshold and the second temperature change rate is equal to or exceeds a second predetermined threshold.
Also disclosed herein is a battery charging method, which method includes providing a current to the battery pack, sensing first and second battery temperatures, determining a first temperature change rate between the first and second battery temperatures, sensing a third battery temperature, determining a second temperature change between the second and third battery temperatures, and disabling termination of the charging method based on a temperature-based scheme if the first temperature change rate is equal to or below a first predetermined threshold and the second temperature change rate is equal to or below a second predetermined threshold.
Additional features and benefits of the present invention are described, and will be apparent from, the accompanying drawings and the detailed description below.